This where automation and scale make a huge difference. A bike can absolutely be more labour intensive to build a single unit than a car, especially when said car is being built by a company with expertise in design for manufacture, and production automation.

Cars a built using huge injection moulded plastics components that are trivial to automate at scale, and metal bodies carefully crafted to make automated assembly trivial. Plus there will be a huge investment into fixtures and assembly processes that effectively reduce the human labour to little more than biological end effectors. Little operator skill or experience is needed for high productivity.

Compare that to bike frames which haven’t been optimised to automated mass production, and where manual welding is primary construction methodology. I can absolutely guarantee that a vaguely welder costs substantially more than someone that just needs to operate a fancy screwdriver. Welding requires significant skill and experience to do productivity, and compensated appropriately. Welders aren’t trivially fungible, screw driver operators are.

If don’t think that labour is the reason for the cost of cargo bikes. Then why don’t make contribution and tells us where you think the cost comes from, and why competitors haven’t sprung up to reduce that cost and make a killing.

I'm sure you could recruit the same bike frame welders in China/Taiwan to make a bike for you. There are some that are willing to make custom modifications on demand for a very reasonable price. If they can do that for one-off welded bikes, then making a standardized cargo frame is absolutely doable for a reasonable budget.

Sure, there's more material, more bending, more welds, and it'll cost more, but you could certainly do that within reason. Like much of the bike industry, these markups are rooted in a foundation of bullshit.